[hoegertn]

Closes #387.

Adds first-class AppSync Lambda BatchInvoke support. Two additive, backward-compatible pieces.

createAppSyncBatchHandler

New factory createAppSyncBatchHandler<T, R, S> in src/lambda/handler.ts, returning an AWSLambda.AppSyncBatchResolverHandler<T, R, S>. Array of events in, R[] out in source order. It mirrors createAppSyncHandler's context and auth plumbing but operates over a batch:

• Authenticates once per batch off events[0] with a fresh AppSyncCognitoAuthorizer, same logger metadata as the single handler.
• Context AppSyncBatchHandlerContext<T, S> extends AppSyncHandlerContext<T> and exposes both ctx.event (= events[0]) and the full ctx.events, so existing handler code that reads ctx.event.arguments keeps compiling.
• Reuses the createAppSyncHandler: support typed errors so expected failures return GraphQL errors without failing the Lambda invocation #385 typed-error model. The inline catch logic was refactored out of createAppSyncHandler into shared toTypedError / buildErrorEnvelope helpers so both handlers run one error path and cannot drift.

maxBatchSize on addLambdaResolver

New optional argument LambdaResolverOptions { maxBatchSize?: number } on GraphQlApi.addLambdaResolver in src/constructs/graphql.ts. When set, the default UNIT resolver gets MaxBatchSize, the BatchInvoke request template MappingTemplate.lambdaRequest('$utils.toJson($context)', 'BatchInvoke'), and the lambdaResult() response template. Unset keeps today's exact single-invoke shape. Combining maxBatchSize with typedErrors throws before any resource is created, since the BatchInvoke templates have no equivalent on the APPSYNC_JS pipeline resolver.

Error semantics

Batch errors are fail-whole-batch by design. A thrown typed error returns one identical envelope per event (AppSync's BatchInvoke contract expects one result per event); a plain Error always rethrows. This matches the single handler. A caller wanting per-event partial success catches inside its own ctx.events.map.

Compatibility

Fully additive. New export createAppSyncBatchHandler and a new optional arg on addLambdaResolver. Existing single-item resolvers are untouched, verified byte-for-byte via Match.absent() assertions on the no-options resolver.

Testing

• npm run build (compile + jest + package): green.
• npm run test: 14 suites, 147 tests pass (+10 new). New tests cover order preservation, once-per-batch auth off events[0], per-event envelope fan-out, rethrow paths, and CDK template assertions for MaxBatchSize + BatchInvoke/lambdaResult templates plus the typedErrors guard.
• npm run eslint: clean.

Notes

The S type parameter is bounded S extends Record<string, any> | null (the upstream default in @types/aws-lambda), because S = any does not compile against AppSyncBatchResolverHandler.

maxBatchSize is forwarded straight to the L2 and is not range-validated at synth time; AppSync caps it at 2000, so an out-of-range value fails at deploy.